1. Field of the Invention
The present invention relates to a precision processing technology of a surface, especially a curved surface, which is capable of processing an optical object or device including the curved surface and a metal mold for molding the optical device precisely and efficiently while reducing variations of the processing precision.
2. Description of the Prior Art
As conventional precision processing technologies for a curved surface of an optical device, for example, there are the ones described in the gazettes of Japanese Patent Laid-Open Nos. Hei 6 (1994)-170763, Hei 7 (1995)-68456 and Hei 11 (1999)-245152.
The technology described in the gazette of Japanese Patent Laid-Open No. Hei 6 (1994)-170763 is a polishing method capable of executing a polishing step while automatically creating a polishing orbit in the same apparatus. In the polishing method, a measuring tool is attached to the processing apparatus, a polishing area of a work is divided in two axial directions perpendicular to each other by an effective radius unit where a polishing tool contacts the work, and with thus obtained divisional lines taken as a polishing pattern, the polishing area is made to profile the pattern. Thus, impedance control is carried out so that force detected by a force sensor can be a set value to obtain a position and data of a curved surface in an intersection point of the divisional lines. Thereafter, the polishing tool is attached, and based on the above-described curved surface data, a polishing route is decided, and thus the polishing is carried out by the impedance control.
Moreover, the technology described in the gazette of Japanese Patent Laid-Open No. Hei 7 (1995)-68456 is a corrective polishing processing method capable of automating polishing processing for a high-precision metal mold, an aspheric lens and the like that is not axially symmetric and of enhancing precision thereof. In the corrective polishing processing method, a self-profile polishing apparatus and an automatic measuring apparatus are disposed on a principal axis head of a NC machine tool controlled by a control apparatus, and measurement and corrective polishing processing are performed for a surface of processed matter. Then, during the measurement, a moving tolerance and a thermal drift of the machine are corrected, an actual polished quantity per one path is monitored based on a result of the measurement, and the next polished quantity is estimated based on this actual polished quantity, and the next polishing number is set. Simultaneously, in order to prevent overshoot due to variations of the polished quantity, a correction coefficient is introduced.
Furthermore, the technology described in the gazette of Japanese Patent Laid-Open No. Hei 11 (1999)-245152 is a polishing apparatus for high-precision finish polishing for a surface shape of a surface of an optical use and for achieving higher efficiency of the polishing processing for a curved surface of an optical object. The polishing apparatus for polishing the curved surface of a work attached to a work support tool, includes: a polishing head provided with a polisher in a tip thereof, the polisher processing the optical curved surface of the work while rotating the same; a slider for pressing the polisher to a normal line direction to the curved surface, the slider having a polishing head disposed thereon; a motor rotating the polisher; a Z-axis mechanism unit for moving the polisher provided in the polishing head disposed in the slider so as to abut against or to be isolated from the curved surface of the work; and a non-contact displacement meter attached to the polishing head and moving together with the polishing head, wherein the curved surface is polished into a desired shape by the polisher while a distance between the curved surface and the non-contact displacement meter being measured in a state where the polisher abuts against the curved surface.
Incidentally, heretofore, as an article processed to have a curved surface, a mirror for X-rays has been processed highly precisely. A final processing step for the mirror is polishing. The mirror for X-rays has a relatively large curvature radius, which is nearly a plane. On the other hand, in the case where a free curved surface of an device in an optical field, a mold (or a piece of the mold) or the like is polished precisely, which has a curvature radius smaller than that of the mirror for X-rays, it has been found out that there occurs a problem of a large tolerance of an actual removal depth with respect to a target removal depth if the curvature radius is reduced (a value of the curvature is isolated from zero) as shown in FIG. 8. Note that, in FIG. 8, a curvature of a convex surface is represented as positive, and a curvature of a concave surface is represented as negative.
Moreover, particularly in a free curved surface in which a large change of the curvature radius such as a change from the convex surface to the concave surface is present, the above-described tolerance greatly affects the processing precision. Accordingly, it has been made apparent that some correction is required in response to the curvature radius. Heretofore, a relationship between the removal depth and the processing condition has been previously grasped, and the processing condition has been set with respect to the target removal depth. However, the relationship described above has not been corrected in response to the shape of the processed surface, and particularly, to the curvature radius.
The related art described in the foregoing gazette of Japanese Patent Laid-Open No. Hei 6 (1994)-170763 is an example of automatically creating the polishing orbit, where the correction for the shape has not been performed. Therefore, high-precision processing cannot be performed.
Moreover, in the high-precision processing according to the related art described in the foregoing gazette of Japanese Patent Laid-Open No. Hei 7(1995)-68456, steps of measuring the shape, polishing the surface, measuring the shape, polishing the surface . . . are iterated plural times, whereby desired precision is achieved taking a long time. Also in this example, the correction for the curvature radius is not performed.
Furthermore, the related art described in the foregoing gazette of Japanese Patent Laid-Open No. Hei 11(1999)-245152 is a method for correcting the position of the polishing head based on measurement data obtained while measuring the distance between the processed surface and the non-contact displacement meter moving together with the polishing head. In this method, since the processed surface is contaminated by cuttings, abrasive grains or the like, the measuring precision has limitations. Accordingly, the processing precision based on the measurement result also has limitations naturally.
As described above, each of the related prior arts has limitations in processing precision, and convergence of the processing precision also has limitations. Therefore, each of the related arts is still insufficient for realizing the high-precision processing while reducing the variations of the processing precision as much as possible.